In multiplexed optical communication networks, a single optical fiber typically carries a signal having multiple independent data channels with each data channel assigned to a different wavelength. Such networks are referred to as wavelength division multiplexed (WDM) networks. In WDM networks, the signal at each of these different wavelengths may be routed to different destinations. However, if a signal has too much power, it may overload a receiver in the network. An attenuator may be used to reduce signal intensity that may otherwise overload a receiver in the network. An attenuator may also be used for other functions such as to block extraneous signals at other wavelengths and to balance signals transmitted through the same system at different wavelengths.
As signals propagate through the optical fiber, the signals may also experience both transmission losses and coupling losses at points along the network. In order to compensate for these losses, WDM networks often include optical amplifiers at various points in the network to amplify the signals. A fiber amplifier may not amplify all wavelengths equally, since fiber amplifier gain typically varies with wavelength. This causes some wavelengths in a WDM signal to become stronger than others.
One problem with prior WDM networks is the amplification gain provided by the amplifiers may be uneven, such that certain wavelengths are amplified to a greater degree than other wavelengths. As the amplified signals experience successive losses and amplification, the variations in intensity between the different wavelengths increases. Such non-uniformity in losses and gains accumulates and causes transmission errors in the form of interchannel crosstalk and data loss unless the signals are equalized. An equalizer may be used to compensate for these differences by transmitting more light at wavelengths where gain is weakest and partially blocking light at wavelengths where gain is strongest.
One prior art solution for WDM equalization is illustrated in FIG. 1. An optical signal received from an input fiber at an input port is expanded into a relatively large beam via a collimating lens. The expanded beam illuminates a plane diffraction grating that operates in a reflection mode and light, being reflected, passes through a focusing lens. The diffraction grating disperses the input light by wavelength and the lens focuses reflected light in an attenuation plane. Different wavelengths are focused into different light spots at the attenuation plane, as shown in FIG. 1. In the attenuation plane, an array of optical modulators are spaced at a pitch such that each modulator receives one of the different wavelength signals.
Each optical modulator causes a signal to be produced that corresponds to the signal it receives, except that the signal strength of the reflected signal is attenuated to fall within a desired range. Each signal reflected by the array of modulators is again directed toward the diffraction grating, with all signals directed by a folding mirror toward a second collimating lens. The second collimating lens combines all the signals into a single optical output fiber at the output port.
One problem with such an equalizer is that the use of separate ports for the incoming and outgoing signals and the use of a large number of components may lead to reliability problems and thermal instability of the system. Another problem with prior equalizers is that the large number and size of the components unnecessarily increase the size and cost of the equalizer. For example, the alignment of a large number of components increases manufacturing time and cost.